1. Field of the Invention
The present invention relates to an oscillator with a differential structure, and more particularly, to an oscillator with a differential structure having a wiring of a transistor to maintain symmetry of a differential signal.
2. Description of the Related Art
An oscillator is a device allows for output of a desired frequency, using a voltage applied from the outside. An oscillator using a transistor is generally used, and there are an LC oscillator, an RC oscillator, and an oscillator using a wien-bridge, which are all sinusoidal oscillators.
An oscillator, an active component for amplifying, includes a transistor and a negative resistor, and in connection with such an oscillator, a voltage-controlled oscillator including transistors cross over each other has been proposed in the related art.
FIG. 1 is a diagram illustrating a common oscillator.
‘MN1’ and ‘MN2’ indicate transistors. As illustrated in FIG. 1, transistors M1 and M2 are arranged in a differential structure and the differential structure generates negative resistor. Further, an inductor and a capacitor in an oscillator are the most essential parts for generation of an oscillation frequency in the entire oscillator. It can be seen in FIG. 1 that a wire connecting a drain electrode of the transistor MN1 and a gate electrode of the transistor MN2 and a wire connecting a gate electrode of the transistor MN1 and a drain electrode of the transistor MN2 cross over each other.
The crossing-over structure illustrated in FIG. 1 is one of the most popular structures for oscillators formed in an integrated circuit. The output of the oscillator illustrated in FIG. 1 is generated as differential signals from the drains of the transistors MN1 and MN2. The frequency of the oscillator can be changed by changing the value of the inductor or the capacitor illustrated in FIG. 1 and the frequency of a VCO (Voltage-Controlled Oscillator) is changed by controlling a voltage.
The transistors MN1 and MN2 in the oscillator illustrated in FIG. 1 are differentially operated. Accordingly, when there is a phase difference of exactly 180 degrees between the voltages or currents at the gate, the drain, and the source of the transistor MN1 and the voltages or currents at the gate, the drain, and the source of the transistor MN2, the oscillator ideally operates.
When a phase difference of exact 180 degrees is not achieved in spite of differential operation inside the oscillator of FIG. 1, the performance of the oscillator decreases, such as reduction of output and an increase of a harmonic frequency. In order to prevent this problem, it is a very important factor to ensure symmetry of nodes where differential signals are generated, in arrangement of transistors and wirings according to the circuit diagram of FIG. 1.
FIGS. 2A to 2C are diagrams illustrating symbols and arrangement of a MOSFET transistor.
FIG. 2A illustrates symbols of the MOSFET transistor. FIG. 2B conceptually illustrates arrangement of the MOSFET transistor illustrated in FIG. 2A, corresponding to the symbols. As in the arrangement of the transistor illustrated in FIG. 2B, electrodes of the transistor are composed of a drain, a gate, and a source, and exactly, there is a need for forming a body-contact for forming body-bias of the transistor, as in FIG. 2B. However, the body-contact will be briefly described with reference to FIG. 2C, under the assumption that it was formed, for the convenience of description.
FIG. 3 is a diagram illustrating wirings for transistors according to the related art, using the arrangement of a transistor illustrated in FIG. 2.
As illustrated in FIG. 3, a first wiring between a gate electrode of a transistor MN1 and a drain electrode of a transistor MN2 and a second wiring between a drain electrode of the transistor MN1 and a gate electrode of the transistor MN2 cross over each other. According to the typical arrangement in the related art, when the gates and the drains of the transistors MN1 and MN2 are connected, there is a region with a crossover. The crossing wires are integrated in different metal layers in an integrated circuit to prevent a short circuit at the crossover.
According to the arrangement of transistors for an oscillator of the related art illustrated in FIG. 3, there is no problem with normal operation of the circuit, but the parasitic components of the highest metal layer and the lower metal layer under the highest metal layer are different. That is, the highest metal layer is far from the substrate of the integrated circuit, so a loss due to the substrate is relatively small, while the lower metal layer is close to the substrate, so a loss due to the substrate is relatively large. Accordingly, in this configuration, impedances produced by the parasitic components at the gates or the drains of the transistors MN1 and MN2 are not symmetric to each other, so the power generated by the transistors is different, and the voltages or the currents are also different. Further, the phases of differential signals generated at two nodes fail to make exactly 180 degrees, such that a harmonic component is generated at an output node.
In order to solve this problem, it may be possible to consider the following wiring type modified from the type described above. That is, a method, which makes a metal wire connecting a gate of a transistor MN1 and a drain of a transistor MN2, using the highest metal layer of a corresponding integrated circuit, makes a metal wire connecting a gate of the transistor MN2 and a drain of the transistor MN1, basically using the highest metal layer, and partially uses a layer under the highest layer for the region with a crossover, may be considered. In this configuration, there is a section where it is required to connect the highest metal layer and the lower layer in order to connect the gate of the transistor MN2 and the drain of the transistor MN1, and in this case, a layer called Hole-Via is generally used.
However, the Hole-Via causes an additional parasitic impedance component. In this configuration, as compared with the wiring between the gate of the transistor MN1 and the drain of the transistor MN2, the metal layer under the highest metal layer is partially used for the wiring between the gate of the transistor MN2 and the drain of the transistor MN1 and the Hole-Via is additionally used, such that there is still the problem of asymmetry between differential signal lines.
The background art of the present invention has been described in Korean Patent Publication No. 2012-0028391 (2012.03.22).